Field of the Invention
The present invention is directed to a process for forming through connections to electrically interconnect two metal layers disposed on opposite surfaces of a substrate such as in a printed circuit board.
To achieve a basic reduction in the size of a printed circuit, the use of a printed circuit board having a circuit pattern applied on opposite surfaces of a substrate has been utilized and to interconnect portions of the two patterns on opposite surfaces of the substrate, a connection extending through the substrate have been provided. In addition to reducing the size of the circuit boards, the use of through contacts produces a plurality of additional advantages. For example, the through contacts firmly secure the circuit patterns of the metal layer onto the substrate and the joining of the leads from components to the circuit patterns formed in the layers is greatly improved by inserting the lead into the aperture of the through contact and soldering the lead to the circuit patterns.
For the production of through contacts, processes are known in which a combination of currentless or galvanic metal deposition and etching steps are employed. In these processes, a substrate of a base material which has a metal layer applied on opposite surfaces is cut to the desired shape. An aperture is formed through the substrate and the opposite surface metal layers, the surface of the aperture is subsequently sensitized and activated to render the surface conducting, metal is deposited on the surface of the aperture by a currentless metal deposition process. The metal layers are then coated with negative patterns of a galvano-resistant dye. The conductor paths and bores are now galvanically provided with a metal structure and a thin, etch resistant final surface is then applied. When the galvano-resistant dye has been removed, the uncovered metal coating is etched away to provide the circuit patterns on the surface of the substrate. Since the through contacts or connections and the conductor paths of the circuit patterns are protected by the etch resistant final surface, they are retained and uneffected by the etching process. Such a procedure was disclosed in a book by H. Schikarski Die gedruckte Schaltung, (The Printed Circuit) Telekosmos-Verlag, Stuttgard 1966, page 60. This known process which can be modified in various ways in expensive and requires a large outlay of money and labor because of the relatively large number of process steps required for the currentless metal deposition. Also, the metal deposited by the currentless deposition acts as a type of dividing surface between the metal layers on the substrate and the galvanically deposited metal. The currentless deposited metal has a lower degree of purity, has a lower ductility and has more poor mechanical qualities.
The problems which occur with the use of a currentless deposition are avoided in other known processes which draw or deform a portion of the metal coating on each surface of the substrate into the aperture and then electrically connecting the drawn coatings together. In German Offenlegungschrift No. 1,640,468, a process for the production of through contacts or connections in a circuit board is disclosed. In this process, a circuit card or board is placed on a hard base and at a predetermined point for the through connection, the overlying conducting metal layer is pierced with a pointed tool so that a portion of the metal layer is cold drawn into a hollow frustrum-shaped projection which penetrates through the base material of the substrate and touches the opposite metal layer. Then, an electrical conductive connection is established at the point of contact between the drawn portion and the opposite layer, for example, by a galvanic metal deposition in the cavity of the hollow frustrum-like projection. This known process is limited to specific thicknesses and materials for the base substrate. For example, if the materials of the substrate have an elastic reformation property, displacement of the base material can create tensions in the material, cracks and breaks in the through contacts or connections. Also, the surfaces of the non-deformed conducting layer which face the substrate cannot be cleaned or can be cleaned only in a poor fashion at the point of the through contact. This means that the reliability and mechanical stability of the through connection or contact is not always obtained.
In German Offlegungschrift No. 1,814,805, a process is described for producing through connections. In this process, a substrate of base material is provided with a single metal layer on one side and is then provided with an aperture. Subsequently, a second metal layer is provided on the other surface of the substrate and is caused to adhere thereto by pressure and temperture. After the production of the conductor paths, the metal lining which was the last to be applied is pressed with eye-pointed needles or tools into the holes to form lugs which are turned over onto the lands of the first layer and soldered into position. Although the through contacts produced in this process are reliable and mechanically stable, their production requires considerable outlay in labor costs and is expensive. Also, it is not possible to use commercially available substrates which are provided with metal layers on both surfaces. Since the temperature used to apply the second layer may cause unavoidable warping of the substrate, the exact positioning of the connection may not be obtained.
U.S. Pat. No. 2,889,393, discloses a method or process of forming through contacts. In this process, a substrate of base material which is free from metal coatings or layers is perforated and then under the effect of pressure and temperture, metal layers are caused to adhere to both sides of the perforated base material. Then at the points of perforation, the portions of the metal layers which are overlying the perforations are deformed into the perforations and into projection with a shape of a frustrum so that they touch one another with a circular zone of contact. The completion of the electrical connection is formed by spot welding the bases of the projection or by providing holes in the circular contact zone and then galvanically depositioning a metal or applying a solder layer. The production of these through contacts or connections is again expensive in terms of effort and cost and it is not possible to use commercially available material provided with the metal layers on both surfaces. It is also difficult to find the apertures or perforations formed in the substrate after the application of the metal layers on opposite surfaces thereof. Also, the possibility of warping of the base material or substrate which would occur during the adhesion of the metal layers under the effect of temperature and pressure can cause considerable errors in the location of the through contacts.